Re: [PATCH v29 23/32] x86/cet/shstk: Add user-mode shadow stack support

Dave Hansen
2021-08-27T20:25:35Z

On 8/27/21 11:21 AM, Borislav Petkov wrote:
> On Fri, Aug 27, 2021 at 11:10:31AM -0700, Yu, Yu-cheng wrote:
>> Because on context switches the whole xstates are switched together,
>> we need to make sure all are in registers.
> There's context switch code which does that already.
>
> Why would shstk_setup() be responsible for switching the whole extended
> states buffer instead of only the shadow stack stuff only?

I don't think this has anything to do with context-switching, really.

The code lands in shstk_setup() which wants to make sure that the new
MSR values are set before the task goes out to userspace. If
TIF_NEED_FPU_LOAD was set, it could do that by going out to the XSAVE
buffer and setting the MSR state in the buffer. Before returning to
userspace, it would be XRSTOR'd. A WRMSR by itself would not be
persistent because that XRSTOR would overwrite it.

But, if TIF_NEED_FPU_LOAD is *clear* it means the XSAVE buffer is
out-of-date and the registers are live. WRMSR can be used and there
will be a XSAVE* to the task buffer during a context switch.

So, this code takes the coward's way out: it *forces* TIF_NEED_FPU_LOAD
to be clear by making the registers live with fpregs_restore_userregs().
That lets it just use WRMSR instead of dealing with the XSAVE buffer
directly. If it didn't do this with the *WHOLE* set of user FPU state,
we'd need more fine-granted "NEED_*_LOAD" tracking than our one FPU bit.

This is also *only* safe because the task is newly-exec()'d and the FPU
state was just reset. Otherwise, we might have had to worry that the
non-PL3 SSPs have garbage or that non-SHSTK bits are set in MSR_IA32_U_CET.

That said, after staring at it, I *think* this code is functionally
correct and OK performance-wise. I suspect that the (very blunt) XRSTOR
inside of start_update_msrs()->fpregs_restore_userregs() is quite rare
because TIF_NEED_FPU_LOAD will usually be clear due to the proximity to
execve(). So, adding direct XSAVE buffer manipulation would probably
only make it more error prone.